Prep v230: Move installed headers

Prep v230: Move installed headers

The headers are now installed into

  /usr/include/elogind/systemd

and the pkg-config file now returns -I/usr/include/elogind for CFLAGS.

Further
  /usr/include/elogind
contains symlinks to the headers, so existing packages already including
<elogind/sd-login.h> will not be broken.

This way no software has to change their include lines anywhere to
support elogind any more. All they have to do is either add
LIBSYSTEMD_CFLAGS or LIBELOGIND_CFLAGS from pkg-config and be done
with it.

So most patching can be avoided, just some configure adaption is
needed.

However, this can lead to systemd/elogind bug confusion if there is anything
mixed up on a system that shouldn't.

Downstreams, chose any of the two ways to your own discretion!

See https://bugs.freedesktop.org/show_bug.cgi?id=101251